This invention relates to optical communications systems and, more particularly, a gain control system for optical amplifiers for use in such systems.
Large capacity optical transmission systems typically combine high speed signals on a signal fiber by means of Wavelength Division Multiplexing (WDM) to fill the available bandwidth. In these WDM optical transmission systems, in general, rare-earth doped fiber optical amplifiers (such as Erbium or Erbium-Ytterbium doped) are used to compensate for the fiber link and splitting losses. Such amplifiers are provided with laser pump light to cause the optical amplification.
To provide both low-noise and high gain in rare-earth doped fiber amplifiers, two lasers (pumps) of different wavelengths may be applied to the amplifying medium, one pump signal (usually the one with the shorter wavelength) propagates with the signal to be amplified (co-propagating), while the other pump signal propagates oppositely the signal to be amplified (counter-propagating).
Gain transients, where step changes in the amplifier gain are caused by variations in input signals, are a major problem for WDM optical systems. Gain transients occur because channels are added or dropped either due to network reconfiguration or failures. Adding channels can depress the power of the present channels below the receiver sensitivity. Dropping channels can give rise to error events in the surviving channels because the power of the surviving channels can surpass the thresholds for non-linear effects. The error bursts in the surviving or present channels as a result of these power transients are unacceptable to service providers. Various other factor also give rise to gain modulation, causing non-uniform amplifier gain.
Some of these effects can be eliminated if the amplifier gain, and hence gain spectrum, is controlled independently of input signal level. In this way, a constant gain can be maintained regardless of the number of channels present at the input. This requires rapid gain control to respond to channel adding and dropping at the input, without giving rise to large or prolonged gain transient effects. Known systems for implementing independent amplifier gain control use automatic gain control (AGC) in the form of opto-electronic or all optical feedback loops for controlling the laser pump source to provide a required change in amplifier pumping. The all-optical option is more desirable in terms of reduced complexity and cost.
AGC schemes may use feedforward or feedback loops, or a combination of these, in order to derive control signals from measures of input and output powers so as to increase the amplifier pump power when more output power is required.
It is also known to provide a series of so-called concatenated amplifier stages within a single optical amplifier. Each stage has its own associated doped fiber section and pump sources. For each individual stage, the noise performance is improved for higher power operation. With this in mind, it has been recognised that the first stage within such an amplifier should be operated at the highest possible power, so that the noise introduced by the first stage, and which is amplified by subsequent stages, is kept to a minimum. Of course, imposing a minimum pump power for the first stage provides a limitation to the dynamic range of the gain control system.
The inventors have recognised a further problem arising with multiple-stage optical amplifiers, particularly when individual stages of the amplifier may be switched in or out of operation depending upon the gain requirement. When an amplifier stage is introduced in response to an increased gain demand, by driving the pump source or sources of that stage, the open loop gain of the amplifier is altered. Accordingly, a fixed processing regime for the input and/or output powers to derive the required pump control will not take into account this varying open loop gain, so that the control loop will not be optimum.
According to the invention, there is provided an optical amplifier comprising:
at least first and second amplifier stages, each stage comprising a doped fiber and a pump source for providing pump light to the fiber;
a power measurement circuit for measuring the input and output power of the amplifier; and
a driver circuit for providing pump control signals for controlling the pump source of each amplifier stage in dependence on the input and output power in order to maintain a substantially constant gain, the power measurement circuit and the driver circuit defining a gain control loop,
wherein the input and output power is processed in order to derive target pump source levels for achieving the substantially constant gain, and wherein the temporal response of the gain control loop is varied in dependence on the target pump source levels.
The gain control loop will have different open loop gain depending upon the target pump source levels, which in turn define the combination and levels of the amplifier stages to be operation. The transfer function of the control loop is altered in dependence on these amplifier settings, and according to the invention the control loop characteristics are altered in response to these changes in open loop gain. The response time of the gain control loop is therefore matched to the open loop gain of the amplifier, taking into account the combination and setting of the amplifier stages which are to be active.
Preferably, the doped fiber of each stage comprises an Erbium doped optical fiber. The power measurement circuit is preferably for measuring the input and output power of the amplifier, so that a constant gain can be achieved.
The driver circuit may generate the target pump source levels by processing an error signal using a digital signal processor, and the target pump source levels then influence the processing of the error signal thereby changing the temporal response of the gain control loop.
The digital signal processor may comprise a PID controller for processing the error signal, and the proportion term is then varied in dependence on the target pump source levels.
The generation of the pump source target levels may be such that a first general pump drive level input to the mapping device gives rise to target pump source levels with the pump source of one amplifier stage turned on and the pump source of the other amplifier stage turned off, and a second general pump drive level input to the mapping device gives rise to target pump source levels with the pump source of both amplifier stages turned on. In other words, a pump control scheme is provided by which the pump sources can be controlled independently to be switched in or out of operation depending on the overall pump power required to maintain the desired gain.
The invention also provides a method of controlling an optical amplifier comprising at least first and second amplifier stages, each stage comprising a doped fiber and a pump source for providing pump light to the fiber, the method comprising;
measuring the power at the input and output of the amplifier;
processing the input and output power in order to derive target pump source levels for achieving a substantially constant gain, the power measurement and pump source control implementing a gain control loop,
wherein the temporal response of the gain control loop is varied in dependence on the target pump source levels.
The AGC loop can be implemented as a DSP, and the invention thereby further provides a storage medium containing computer executable instructions for processing two inputs representing an input and an output power of an optical amplifier comprising at least first and second amplifier stages, the instructions implementing a method comprising the steps of:
calculating a target output power from the input power;
obtaining an error signal representing the difference between the target output power and the output power;
processing the error to derive a target pump level for the amplifier, and deriving individual target pump source levels for the individual amplifier stages from the target pump level,
deriving individual pump source control signals to achieve the target pump source levels,
wherein the processing of the error takes into account the obtained target pump level.
The error may be processed using a PID controller, and the proportional control of the PID controller being weighted in dependence on the target pump level, taking into consideration the individual target pump source levels corresponding to the target pump level.